The invention generally relates to a method of operating a gas- and steam-turbine installation. Preferably, in the method the flue gas discharging from a gas turbine which can be operated with both gas and oil is directed via a heat-recovery steam generator. The heating surfaces of the generator are preferably connected in a water/steam circuit of a steam turbine having a number of pressure stages, with condensate preheated in the heat-recovery steam generator being heated as feedwater, under high pressure compared with the condensate, and being fed as steam to the steam turbine.
In a gas- and steam-turbine installation, the heat contained in the expanded working medium or flue gas from the gas turbine is utilized for generating steam for the steam turbine connected in a water/steam circuit. In this case, the heat transfer is effected in a heat-recovery steam generator or boiler which is connected downstream of the gas turbine and in which heating surfaces are arranged in the form of tubes or tube bundles. The latter in turn are connected in the water/steam circuit of the steam turbine.
The water/steam circuit in this case normally comprises a plurality of pressure stages, for example two or three pressure stages, a preheater and an evaporator and also a superheater being provided as heating surfaces in each pressure stage. EP 0 523 467 B1, for example, discloses such a gas- and steam-turbine installation.
In this case, the total water quantity directed in the water/steam circuit is proportioned in such a way that the flue gas leaving the heat-recovery steam generator, as a result of the heat transfer, is cooled down to a temperature of about 70xc2x0 C. to 100xc2x0 C. Thus, in particular, the heating surfaces exposed to the hot flue gas and pressure drums provided for a water/steam separation are designed for full-load or rated operation, at which an efficiency of currently about 55% to 60% is achieved.
For thermodynamic reasons, it is also desired in this case that the temperatures of the feedwater, which is directed in the heating surfaces and is under varying pressure, are as close as possible to the temperature profile of the flue gas cooling down along the heat-recovery steam generator as a result of the heat exchange. The aim here is to keep the temperature difference between the feedwater directed via the individual heating surfaces and the flue gas as small as possible in each region of the heat-recovery steam generator. Thus, as high a proportion as possible, of the heat quantity contained in the flue gas, is transformed in the process. A condensate preheater for heating condensed water from the steam turbine is additionally provided in the heat-recovery steam generator.
The gas turbine of such a gas- and steam-turbine installation may be designed for operation with various fuels. If the gas turbine is designed for fuel oil and for natural gas, fuel oil, as fuel for the gas turbine, is only provided for a short operating period, for example for 100 to 500 h/a, as xe2x80x9cbackupxe2x80x9d for the natural gas. The priority in this case is normally to design and optimize the gas- and steam-turbine installation for natural-gas operation of the gas turbine. As such, a sufficiently high inlet temperature of the condensate flowing into the heat-recovery steam generator is then ensured during fuel-oil operation. In particular, during a change from gas operation to oil operation, the necessary heat can be extracted from the heat-recovery steam generator itself in various ways. One possibility is to bypass the condensate preheater entirely or partly and to heat the condensate in a feedwater tank, connected in the watertsteam circuit, by feeding low-pressure steam. However, such a method, at low steam pressures, requires a large-volume and possibly multi-stage heating-steam system in the feedwater tank, a factor which, during long heating intervals, may put at risk deaeration normally taking place in the feedwater tank.
In order in particular to ensure effective deaeration, the condensate temperature in the feedwater tank is normally kept within a temperature range of between 130xc2x0 C. and 160xc2x0 C. In this case, preheating of the condensate via a preheater fed with low-pressure steam or hot water from an economizer is provided as a rule, so that the heating interval of the condensate in the feedwater tank is kept as small as possible. In this case, in particular in dual- or triple-pressure installations, hot-water extraction from the high-pressure economizer is necessary in order to provide sufficient heat. However, this has the considerable disadvantage, in particular in triple-pressure installations or circuits, that an external, additional condensate preheater, which has to be designed for the high pressures and high temperatures or high temperature differences, is required. This method is therefore already extremely undesirable on account of the considerable costs and the additional space required for the condensate preheater.
It is also possible, during oil operation of the gas turbine, to carry out or assist the condensate heating in the feedwater tank or in the de-aerator with a partial flow from a reheater. However, this method also cannot be used in particular in modern installation circuits without a feedwater tank and without a de-aerator, especially as there are no devices or apparatus for mixed preheating.
DE 197 36 889 C1 has certainly disclosed a method which, compared with the methods described, can be carried out with little outlay in terms of apparatus and operation and which is based on a displacement of the exhaust-gas heat in the direction of the condensate preheating as a result of a reduction in the low-pressure range and on an installation of economizer bypasses on the water side. However, there are also limits to the implementation of this method with certain requirements.
An object of an embodiment of the invention is to specify a method of operating a gas- and steam-turbine installation, which method, with at the same time little outlay in terms of apparatus and operation, in an effective manner which is favorable with regard to the efficiency, ensures a change from gas operation to oil operation of the gas turbine while covering a wide temperature range of the inlet temperature of the condensate flowing into the heat-recovery steam generator. Furthermore, a gas- and steam-turbine installation which is especially suitable for carrying out the method is to be specified.
With regard to the method, an object may be achieved according to an embodiment of the invention. To this end, provision is made for feedwater which is under high pressure compared with the condensate and has a high temperature compared with the condensate to be expediently admixed with the cold condensate without a heat exchanger and thus directly via an additional pipeline. The heated feedwater or hot water is extracted as a first partial flow from a high-pressure drum in the case of dual-pressure system, i.e. in the case of a dual-pressure installation, and from the high-pressure drum and/or from an intermediate-pressure drum in the case of a triple-pressure system or triple-pressure installation. Alternatively, the first partial flow may also be extracted at the outlet of the high-pressure economizer or the intermediate-pressure economizer.
If and when required, the pressure of the low-pressure system may be additionally increased in order to displace heat contained in the flue gas from the low-pressure system toward the condensate preheater arranged downstream of the latter on the flue-gas side. It is essential in this case that the heated feedwater, which is extracted from the water/steam circuit at a suitable point and is in the form of a partial-flow mixture of feedwater partial flows of different temperature, is admixed with the cold condensate without prior heating, i.e. without heat exchange in an additional heat exchanger.
In this case, an embodiment of the invention may be based on the idea that an additional heat exchanger which cools the heated feedwater or heating water, extracted from the water/steam circuit, to the temperature level of the condensate system before its pressure is reduced, in order to thereby prevent the generation of steam following the pressure reduction can be dispensed with if a partial flow of feedwater having a likewise high pressure but a comparatively low temperature is admixed with the heated feedwater before its pressure is reduced such that the mixing temperature which occurs is below the boiling temperature in the condensate system.
In this case, in particular in a triple-pressure system, heated feedwater can be extracted from the intermediate-pressure system, from the high-pressure system or from both systems. The extraction here depends essentially on the heat required for heating the condensate and also on which installation efficiency is to be at least maintained during oil operation, serving only as backup, of the gas turbine.
With regard to the installation, the object may be so that the partial-flow mixture formed from the first partial flow of heated feedwater and from the second partial flow of comparatively cool feedwater is admixed with the cold condensate directly and thus without a heat exchanger during a change of operation from gas to oil. The installation comprises a feed line for the heated feedwater, this feed line being directed to the condensate preheater and having an admixing point for feeding the comparatively cool feedwater.
The advantages achieved with embodiments of the invention include, in particular, the fact that a water inlet temperature which is required during oil operation of the gas turbine and is increased compared with the gas operation of the gas turbine, can be set in the heat-recovery steam generator especially simpley, even without an additional heat exchanger or external condensate preheater. It is done by heated feedwater which is set to a suitable mixing temperature and is under high pressure being admixed with the cold condensate directly, i.e. without a heat exchanger.
In this case, by the provision of a partial-flow mixture from two feedwater partial flows of different temperature, a mixing temperature of the partial-flow mixture admixed directly with the cold condensate during oil operation can be produced in an especially simple and effective manner. The mixing temperature is below the boiling temperature of the preheated condensate or of the condensate to be preheated. In addition, since the rate of flow in the condensate preheater correspondingly increases via the returned feedwater, condensate circulating pumps hitherto necessary may be dispensed with. In particular, it is possible to cover a wide temperature range of the inlet temperature of the steam generator or boiler without circuit modification.
It can be seen that the capacity reserves of the high-pressure feedwater pump can also be utilized in this way. This can occur since, during oil operation as compared with gas operation, on account of a lower gas-turbine output, lower delivery quantities are normally also required. Standardization is also possible on account of the operating range expanded in terms of the circuit in an especially effective manner. Furthermore, the investment costs are especially low.
On account of the comparatively less complex controls and changeovers, a comparatively simple mode of operation is achieved on the one hand. Further, comparatively high reliability is also achieved, since components which are less active overall are required. On account of the comparatively small number of components, the maintenance cost is reduced and fewer spare parts are required to be held in stock.